1. Field of the Invention
The present invention relates to downhole drilling seismology and more particularly to apparatus, systems, and methods for implementing seismic and sonic sensors in downhole drill strings.
2. Background
The primary aim of seismic exploration for new oil reserves and development of known reserves is determining the location, shape, and type of reflectors that exist in subterranean formations. A reflector is generally any feature in the formation where there is a change in acoustic impedance. Reflectors may include boundaries between different sedimentary formations, faults, cracks, cavities, zones permeated with different fluids or gases, and zones exhibiting a gradient in pore pressure.
In a conventional surface seismic survey, both sources and receivers are positioned at or near the surface. A source may include a device such as a mechanical wave generator, an explosive, or an air gun, to create seismic or sonic waves to travel through the earth. When the waves bounce off an underground reflector, they are detected by receivers adapted to detect phenomena such as velocity, acceleration, or fluid pressure. Receivers may include, for example, geophones, accelerometers, hydrophones, or similar devices. Seismic survey equipment may be used to synchronize the sources and receivers, record pilot signals representative of the source, and record reflected waveforms detected by the receivers. The recorded data may then be processed to graphically display the time needed for seismic waves to travel between the surface and underground reflectors. If the velocity of seismic waves in each subterranean layer may be determined, the position of each reflector may be calculated.
Although surface seismic surveys are the most widely used type of geophysical survey, they are hindered by noise, interference, and attenuation that may occur at or near the surface. Another disadvantage of surface seismic surveys is their inability to determine the velocity of seismic waves traveling through the underground formations. These velocity measurements are needed to accurately transform the subsurface seismic map from the time domain to the spatial domain. To obtain these measurements, a wireline tool comprising a seismometer is typically lowered into a borehole. In some cases, the seismometer is clamped against the formation inside the borehole to improve the quality and accuracy of measurements taken.
Although a wireline seismic survey may be used to obtain accurate seismic data, this type of survey typically requires lengthy and expensive interruptions of the drilling process, thereby increasing the non-productive time (NPT) of a drilling operation. To avoid NPT associated with wireline surveys, measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools have been developed. However, MWD and LWD tools generally communicate with the surface via mud-pulse telemetry, which is usually limited to the order of 10 bits/second. Due to these data rate limitations and the resulting inability to send complete waveforms to the surface in real time, the development of MWD tools for gathering and transmitting seismic data to the surface has been limited. Furthermore, because of the possibility that the drill string may become stuck in the borehole, MWD seismic tools that have been developed are typically not clamped inside the borehole, unlike their wireline counterparts. This may result in a relatively poor coupling with the borehole and may reduce the quality and utility of seismic data gathered by these tools.
Accordingly, what are needed are improved apparatus, systems, and methods for gathering seismic data while drilling. More particularly, apparatus, systems, and methods are needed for clamping seismic receivers to the formation inside the borehole, while reducing the possibility that the drill string will become stuck while drilling. Further needed are apparatus, systems, and methods that are able to isolate seismic receivers from vibrations propagating through the drill string. Further needed are apparatus, systems, and methods for calculating the divergence and curl of seismic waves, thereby enabling the differentiation of compression waves from shear waves. Such apparatus, systems, and methods are disclosed herein.